As methods for forming coating films on automobile bodies, a 3-coat 2-bake process and a 2-coat 2-bake process have been widely used. The 3-coat 2-bake process comprises the following steps in order: after subjecting a substrate to electrodeposition coating and curing by heating, (1) application of an intermediate coating composition→(2) curing by heating→(3) application of a base coating composition→(4) preheating (preliminary heating)→(5) application of a clear coating composition→(6) curing by heating. The 2-coat 2-bake process comprises the following steps in order: after subjecting a substrate to electrodeposition coating and curing by heating, (1) application of an intermediate coating composition→(2) curing by heating→(3) application of a top coating composition→(4) curing by heating.
Generally, the 3-coat 2-bake process is used for forming a coating film of a so-called metallic color by using a base coating composition comprising an effect pigment, whereas the 2-coat 2-bake process is used for forming a coating film of a so-called solid color, such as white or black, by using a top coating composition comprising a color pigment.
However, in recent years, for the purpose of saving energy, consideration has been given to omission of the heat-curing step that is performed after applying the intermediate coating composition, and research has been conducted on a 3-coat 1-bake process comprising the following steps in order: (1) application of an intermediate coating composition→(2) preheating (preliminary heating)→(3) application of a base coating composition→(4) preheating (preliminary heating)→(5) application of a clear coating composition→(6) curing by heating, and on a 2-coat 1-bake process comprising the following steps in order: (1) application of an intermediate coating composition→(2) preheating (preliminary heating)→(3) application of a top coating composition→(4) curing by heating.
From the viewpoint of minimizing environmental pollution caused by volatilization of organic solvents, particular demand exists for a 3-coat 1-bake process or a 2-coat 1-bake process using aqueous coating compositions as the intermediate coating composition, base coating composition, and top coating composition.
However, in the 3-coat 1-bake process using an aqueous intermediate coating composition and an aqueous base coating composition, and in the 2-coat 1-bake process using an aqueous intermediate coating composition and an aqueous top coating composition, the resulting coating film may have insufficient water resistance due to the use of a water-soluble or water-dispersible resin, as well as insufficient smoothness and distinctness of image due to the formation of a mixed layer between the aqueous intermediate coating composition and the aqueous base coating composition, or between the aqueous intermediate coating composition and the aqueous top coating composition. These have been problems that require solutions.
For example, Patent Literature (PTL) 1 discloses that in a coating method using a thermosetting aqueous intermediate coating composition (A), a thermosetting aqueous base coating composition (B), and a thermosetting clear coating composition (C), the method comprising applying the thermosetting aqueous intermediate coating composition (A) and the thermosetting aqueous base coating composition (B) by a wet-on-wet process, when the base resin of the aqueous intermediate coating composition (A) has a neutralization value of 10 to 40 mg KOH/g and the base resin of the aqueous base coating composition (B) has a neutralization value that is greater than that of the aqueous coating composition (A) by 10 to 20, finish appearance such as gloss or distinctness of image is not impaired. The multilayer coating film obtained by this coating method, however, may have insufficient smoothness and water resistance.
Patent Literature (PTL) 2 discloses that in a coating film-forming method comprising successively forming, on an electrocoated substrate, an intermediate coating film, a metallic base coating film, and a clear coating film by using an aqueous intermediate coating composition, an aqueous metallic base coating composition, and a clear coating composition, respectively, when the aqueous intermediate coating composition comprises an aqueous dispersion of amide group containing acrylic resin particles having a particle size of 0.01 to 1.0 μm, which is obtained by emulsion-polymerizing an amide group-containing ethylenically unsaturated monomer and another ethylenically unsaturated monomer, bleeding or inversion at the interfaces of each of the coating film layers can be controlled, and a multilayered coating film with excellent appearance can be formed. However, the multilayer coating film obtained by this coating film-forming method may be insufficient in terms of smoothness, adhesion, and chipping resistance (resistance to coating damage (scratches, peeling, etc.) due to flying stone that hit vehicles during driving, or other factors).
Patent Literature (PTL) 3 discloses that in a multilayer coating film-forming method comprising: (1) a step of providing an electrocoated substrate; (2) a step of applying an aqueous intermediate coating composition to the electrocoated substrate to form an intermediate coating film; (3) a step of successively applying an aqueous base coating composition and a clear coating composition to the uncured intermediate coating film by a wet-on-wet process to form a base coating film and a clear coating film thereon, and (4) simultaneously bake-curing the intermediate coating film, base coating film, and clear coating film, when the aqueous intermediate coating composition comprises a specific acrylic resin emulsion and a specific urethane resin emulsion, and the intermediate coating film formed of the intermediate coating composition has a specific water absorption rate and a specific water elution rate, the formation of a mixed layer between the intermediate and base coating layers can be effectively prevented, and a multilayer coating film having excelling surface smoothness can be formed. However, the multilayer coating film obtained by this method may also be insufficient in terms of smoothness, distinctness of image, and adhesion.
Patent Literature (PTL) 4 discloses that when the intermediate coating material used in a 3-coat 1-bake process is an aqueous intermediate coating composition comprising both a copolymer resin emulsion and a curing agent, the copolymer resin emulsion being one prepared by emulsion-polymerizing (a) a monomer component containing at least one monomer selected from alkyl (meth)acrylates and, if necessary, at least one monomer selected from the group consisting of styrenic monomers, (meth)acrylonitrile, and (meth)acrylamide, (b) a polymerizable unsaturated monomer having an acid group, (c) a polymerizable unsaturated monomer having a hydroxy group, and (d) a crosslinking monomer, the resin having a glass transition temperature of −50 to 20° C., an acid value of 2 to 60 mg KOH/g, and a hydroxy value of 10 to 120 mg KOH/g, curing reactability of the copolymer resin emulsion with the curing agent is enhanced to provide a multilayer coating films with excellent chipping resistance and water resistance, as well as excellent finish appearance. However, the multilayer coating film obtained by this multilayer coating film-forming process may also be insufficient in terms of smoothness and distinctness of image.
Further, Patent Literature 5 discloses that in a coating film-forming method comprising successively forming on a substrate an intermediate coating film, a base coating film, and a clear coating film by a wet-on-wet process, when an intermediate coating composition for forming the intermediate coating film and a base coating composition for forming the base coating film comprise an amide group-containing acrylic resin and a curing agent, and the curing agent contained in the intermediate coating composition comprises an aliphatic isocyanate active methylene blocked isocyanate, and the aliphatic isocyanate active methylene blocked isocyanate has an average functionality of more than 3, the amide group-containing acrylic resin exerts a viscosity-controlling effect and bleeding or inversion at the interfaces between each of the coating layers can be controlled. Further, the use of an aliphatic isocyanate active methylene blocked isocyanate that has excellent low-temperature curability as the curing agent initiates curing of the intermediate coating film earlier than curing of the base coating film and the clear coating film, and also secures sufficient flowability, thus providing excellent substrate hiding power for hiding the surface roughness of an electrodeposition coating film and thus providing a multilayer coating film with excellent finish appearance and excellent coating film properties, particularly excellent chipping resistance.
However, in the multilayer coating film-forming method disclosed in Patent Literature (PTL) 5, the use of aqueous coating compositions as the intermediate coating composition and base coating composition may result in reduced smoothness and distinctness of image of the resulting multilayer coating film due to the formation of a mixed layer between the layers of the intermediate and base coating compositions, or the resulting multilayer coating film may have insufficient chipping resistance.